Light emitting diodes (hereinafter also referred to as LED chips) are light emitting devices (light emitting elements) that act as light sources upon application of a voltage and utilize light emitted as a result of recombination between electrons and positive holes in the vicinity of a contact surface (pn-junction) between two semiconductors. These light emitting devices are small in size and have a high conversion efficiency of electric energy into light, and therefore these light emitting devices are widely used as household electrical appliances, interior illuminations, lighted operation switches, and LED indicators (LED displays).
Differing from electric lamp bulbs using filaments, the light emitting diodes are semiconductor devices, are thereby free from blowout, are excellent in initial drive performance, and have excellent durability even under vibrations and/or repeated ON/OFF operations. They are therefore also used as backlights of indicators or displays typically for automobile dashboards. Particularly, since they can emit light of a clear color with high color saturation without being affected by sunlight, the uses of the light emitting diodes will be expanded even to, for example, displays arranged outdoor, displays for traffic use, and traffic signals, or the like.
As conventional light emitting apparatuses mounted with light emitting devices such as LED chips, for example, there is proposed a light emitting apparatus shown in FIG. 2 (see, for example, Patent Document (Japanese Patent No. 3,316,838)). The light emitting apparatus 1 comprises a ceramic package 3, a LED chip as a light emitting device, a first metal layer 6, a second metal layer 7, and a resin molding 8. The ceramic package 3 includes conductive interconnections (conduction wirings) 2 and has a number of concave openings integrally formed to the ceramic package. The LED chip 5 is electrically connected to the conductive interconnections 2 via bonding wires 4 in the concave opening. The first metal layer 6 and the second metal layer 7 are arranged on a side wall of the concave opening. The resin molding 8 seals the concave opening.
The patent document mentions that, according to the conventional light emitting device, the first metal layer 6 arranged in the concave opening acts to increase the adhesion with the ceramic package 3, and, additionally, the second metal layer 7 acts to reflect light emitted from the LED chip 5, whereby the light loss can be reduced and the contrast typically in displays can be increased.
The conventional light emitting apparatus, however, has a fatal defect of very poor heat radiation performance, since the ceramic package mounted with the LED chip comprises a ceramic material mainly composed of alumina (Al2O3) having a low thermal conductivity of about 15 to 20 W/mK, and the molding resin for sealing the LED chip also has a low thermal conductivity. The LED chip may be broken due to heat generated upon application of a high voltage and/or a large current. Consequently, there has been raised a problem that the conventional light emitting apparatus has a low luminance, since the highest voltage that can be applied to the LED chip is low and the current to be supplied is limited to several ten milliamperes (mA).
In this connection, in the above conventional light emitting apparatus, since the luminance to be technically demanded was low, the conventional light emitting apparatus using a LED chip has been practically used without significant problems even at the above-mentioned current quantity. However, with recent expanding specific uses (application field) of LED light emitting apparatuses, technical demands have been made to achieve structures that can increase the current to be passed to about several amperes at a higher power and can thereby increase the luminance.
Further, in the above conventional light emitting apparatus, the ceramic package integrally formed with a number of concave openings for accommodating the light emitting elements is used, so that there has been posed problems such that a process of manufacturing the light emitting apparatus becomes complicate, and finished accuracy of parts constituting the apparatus is low whereby a sufficient emission property (light emitting property) cannot be obtained. That is, a working operation for integrally forming the number of the concave openings to a hard and brittle ceramic material was an extremely difficult, so that a great working expense and processing charge were required.
On the other hand, in a case where the number of the concave openings were integrally formed to a ceramic member by drilling work at a stage of soft molded body and then the molded body was sintered, a dimension accuracy, a finishing accuracy and a dispersion of surface roughness of the concave openings were disadvantageously deteriorated due to shrinkage error and non-uniformity in material composition, whereby there was posed a problem that an aimed light-reflecting property could not be obtained.
A side surface of the concave opening for accommodating the above light emitting element functions as a reflector for reflecting the emitted light. Since this reflector is integrally formed to the ceramic substrate, a surface roughness Ra of an inner wall surface of the reflector becomes coarse to be about 0.5 μm, so that there was also posed a problem that a scattering and dispersion of the light was liable to occur.
In addition, even if a predetermined inclined angle was tried to be imparted to the inner wall of the reflector so as to control a reflecting direction of the light, a fluctuation or a dispersion of the inclined angle was large, so that it was difficult to stably impart a predetermined inclined angle. At any rate, it was difficult to correctly control the shape accuracy of the concave openings. Furthermore, even if a worker tried to work and adjust the reflector so as to realize a predetermined finishing accuracy, the ceramic material having hard and brittle properties was difficult to be smoothly worked, so that there was also posed a problem that a man-hour required for the working was greatly increased.
Additionally, in conventional light emitting apparatuses as shown in FIG. 2, the LED chip and the conductive interconnections are electrically connected by a wire bonding process, so that the wire and an electrode pad disposed on the LED chip partially block or intercept the emitted light, whereby there was also posed a problem that a light-extraction efficiency was decreased.
Further, a portion where the bonding wire rises protrudes in a thickness direction of the apparatus, and a large electrode region for connecting the edge of the bonding wire is disadvantageously required. Thus, there has been posed a problem that the LED package including the interconnection structure becomes large in size.
Furthermore, when the LED chip is mounted and accommodated in a concave opening as shown in FIG. 2 so as to avoid the adverse influence of the bonding wire protruding in a thickness direction of the apparatus, the light emitted from the LED chip is absorbed by the inner wall of the concave opening to increase the light loss and thereby to decrease the luminous efficiency. Thus, according to the conventional technique, two metal layers that reflect light are arranged on the inner wall of the respective concave openings to thereby reduce the absorption loss of the light.
However, it is very difficult to form such a reflecting metal layer uniformly in the concave opening having a curved inner wall, and the emitted light is partially absorbed by the inner wall to invite light loss. In addition, there has been also posed another problem that the inner wall of the concave opening itself has such a structure as to inhibit the travel or transmission of the light, and the luminance is thereby decreased.